TBH, I don't find Hong Kong to be that bad for buying clothes. There always seems to be a sale on, and the range here is decent.
I think Olympic City is a fairly 'normal' shopping centre by British standards.
Places like H&M, Zara, Hollister, Forever 21, Pull&Bear, American Eagle have a fairly good selection with 'western styles' with reasonable to reasonable-ish prices. You can find them in many parts of Hong Kong. If you're a bit more traditional then M&S had a good range, but tends to be pricier than the ones I've listed above. Also, you can try some sports shops for casual wear. But I find lots of companies (Adidas and Nike) have 'Asian cut' styles here, which aren't very generous on the Western frame.
I'm not sure I'd recommend going to some of the hidden away shopping malls in Mong Kok. They tend to be very 'local' in style and sizing. Unless, of course, someone can recommend somewhere specific. My experiences have all been rather underwhelming.
On the suit front, I looked for one here and found it to be pricer than expected. I ended up getting one from Vietnam for less than half the price, and it is good quality for the price paid. I don't think G2000 is too bad for suits either. Stay away from their slim-fit styles though, as they'll be far too narrow on the shoulders of most Westerners.